The invention is a method for automatic braking, without any active braking operation by the driver, of a vehicle equipped with an electronically controlled braking device which has a service brake, which can be operated electrically and to which a service braking pressure can be applied, and a parking brake, which can be operated electrically and to which a parking braking pressure can be applied. Starting from a state in which the vehicle is stationary as a result of the service brake being applied, the service brake is released and the parking brake is applied as a function of a release criterion.
In conjunction with electronic braking systems (EBS), there are automatic braking methods, such as hillholding, brake-hold and release aid, automatic starting aids or rolling inhibitors, which keep a vehicle stationary after stopping, automatically using the service brake, without the driver having to operate the brake pedal. The service brake is then released automatically again only when the driver starts to drive again by operating a control device such as the gas pedal or clutch pedal, for example, or by selection of a gear. In this case, the vehicle is generally held stationary by the electrically operated, pneumatic service brake. However, this requires energy all the time in the form of compressed air and electrical power. When operating continuously without the engine operating, there is a tendency for the brake to be released, or at least for the braking force to decrease, after a certain amount of time. Alternatively, the parking brake can be operated in order to keep the vehicle in the braked state, as is proposed by way of example in German Patent Document DE 3 521 486 where it is electrically operated. In the case of commercial vehicles, the parking brake is formed by a spring energy storage brake with an energy storage spring which acts against a piston to which compressed air can be applied, with the spring energy storage brake being released by application of air, or being applied by evacuation.
However, this has the disadvantage that the parking brake has a poor dynamic response and requires a relatively large amount of air.
Another conventional method is known from German Patent Document DE 102 38 870 A1. In this case, the service brake is used to produce a braking torque which keeps the vehicle stationary, with the parking brake being operated automatically, and the service brake released, once a holding time interval has passed. This means that the parking brake is applied whenever the stationary period is longer than the predetermined holding time. By way of example, this is disadvantageous when the holding time is shorter than the time during which the vehicle is stationary, for example when stopped at traffic lights, at level crossings when the barriers go down, or on driving onto a road with priority, with a large amount of traffic on it. This is because the parking brake, which has a relatively poor dynamic response, is then applied and must be released again in order to drive away, which takes a certain amount of time and consumes a large amount of compressed air. In order to keep the reaction time as short as possible, relatively large application of air cross sections and a very powerful compressed-air supply must therefore be provided.
German Patent Document DE 103 36 611 A1 discloses a method in which, after previous manual activation of a rolling braking function, on identification that the vehicle is stationary, the braking pressure of the service brake is first of all maintained at the current level. When the driver switches off the voltage supply by means of the ignition switch, or in the event of a defect in the service brake, the parking brake is activated automatically.
In the case of a method according to German Patent Document DE 101 04 498 A1, an automatic changeover takes place from the service brake to the parking brake after activation of a hill-holder function by increasing the braking pedal force by the driver, and after the ignition has been switched off.
However, the cited methods require the driver to carry out active actions (activation of the rolling braking function and/or increasing the pedal force), which, however, the driver may neglect or fail to do, in consequence not resulting in the desired automatic changeover from the service brake to the parking brake.
German Patent Document DE 198 49 799 A1, which forms the prior art in this context, discloses a method in which the parking brake is automatically applied and the service brake is released after a long-term parking condition is satisfied, which is regarded as having been satisfied by the ignition being interrupted.
In contrast, embodiments of the present invention develop a method of the type mentioned above such that the automatic changeover from the service brake to the parking brake can be used for further functionalities.
The exemplary embodiments of the invention provides systems and methods by which when a vehicle is being kept stationary by the service brake, the switching off of the vehicle drive or vehicle engine represents the primary release criterion that is used as the basis for automatic release of the service brake and application of the parking brake. This is because during normal operation, the engine produces not only sufficient compressed air but also sufficient electrical power in order to allow the service brake to be operated safely. In this case, the application of the parking brake in response to the release criterion is initiated without any delay, that is to say without the holding time described in German Patent Document DE 102 38 870 A1. This has the advantage that, during operation of the vehicle, that is to say when the engine is running, the time required to release the brake is relatively short, since the service brake has a comparatively good dynamic response. In consequence, the parking brake is engaged only when it is expected that the vehicle will be stationary for a relatively long time, that is to say after the vehicle drive or engine has been switched off.
According to a further exemplary aspect of the invention, the initiation criterion for the automatic release of the service brake, which then follows this, and application of the parking brake occurs when the service braking pressure falls to a critical braking pressure limit value. Furthermore, as an initiation criterion for the automatic release of the service brake and application of the parking brake, the invention is also used in the situation in which the service brake is not operable, for example because of a defect. In neither exemplary case would the service brake be able to prevent the vehicle from rolling. A person skilled in the art will understand that the magnitude of this critical braking pressure limit value is different depending on the surrounding circumstances, for example to the nominal supply pressure, on the braking cylinder volumes, etc.
In addition, the situation in which the vehicle power supply system voltage has fallen to a critical voltage limit value is also intended to be used, according to a further exemplary aspect of the invention, as an initiation criterion for automatic release of the service brake and application of the parking brake. This is because, when the vehicle power supply system voltage is too low, it is no longer possible to release or apply the electrically controlled service brake, as a result of which total failure of the vehicle brake is prevented by producing appropriate control commands in the controller, by making use of the residual voltage that is still adequate, in order to initiate the automatic release of the service brake and the application of the parking brake, thus preventing inadvertent rolling of the vehicle.
Furthermore, the exemplary situation in which the parking brake can no longer be applied manually because of a defect can be used as an initiation criterion for the automatic release of the service brake and application of the parking brake. A defect in the control device for the parking brake and/or in the way in which its signals are passed on can be detected by suitable sensors.
Operation of a control element for activation of the parking brake may represent an initiation criterion for the automatic release of the service brake and application of the parking brake, in which case, in contrast to the teaching of German Patent Document DE 102 38 870 A1, there is no need to wait for a holding time to pass before the application of the parking brake is initiated. This is advantageous in particular in situations in which the vehicle starts to roll despite the service brake being applied, in some circumstances due to being defective, as a consequence of which the parking brakes must respond very quickly.
The exemplary method steps according to the invention are preferably initiated and controlled by a common electronic controller for the service and parking braking system, or by using the parking braking system being controlled electronically, separately from the service braking system, in communication with the electronic control for the EBS braking system.
According to an exemplary embodiment, starting from the state in which the parking brake is applied and the service brake is released with the vehicle drive switched off, the service brake is automatically applied when the vehicle drive is restarted, and the parking brake is automatically released. In consequence, those method steps which have led to automatic braking and stopping of the vehicle are passed through in the opposite sequence on restarting, with the applied service brake preventing the vehicle from rolling. In this case, the process of releasing the parking brake is preferably initiated only when the service brake is applying a certain minimum braking effect. In contrast, this improves safety since the vehicle will not roll during the changeover from the parking brake to the service brake. When starting off, that is to say when sufficiently high drive torque is present in the drive train, the service brake is then finally automatically released.
In an analogous manner, during the changeover from the service brake to the parking brake after the vehicle drive has been switched off, the time at which the braking effect of the parking brake has reached a specific minimum value with sufficiently high confidence level is preferably in fact used to reduce the braking effect of the service brake to match the increase in the braking effect of the parking brake. Once the parking brake has reached its maximum braking effect or braking force, the service brake is switched off. Finally, once the parking brake has been applied, the service braking control and the parking braking control are switched off.
The release and application of the service brake and of the parking brake in opposite senses when the vehicle drive is switched off and started again are preferably controlled by a characteristic. In this case, by way of example, the braking effect which is produced by the energy storage spring of the parking brake is determined on the basis of the instantaneous pressure, the time which has passed or other measured variables, by using an estimation algorithm. This estimation algorithm may include a delay element since there is generally a certain delay between the signal for the controller and the reaction in the braking cylinder. In consequence, there is no need for any stepped open-loop or closed-loop control of the pressure for the parking braking pressure.
In the situation when the braking effect of the parking brake is not sufficient to prevent the vehicle from rolling freely, the service brake is, for example, once again applied, and an audible and/or visual warning signal is produced. According to another exemplary embodiment of the invention, when the service brake is used for braking before the vehicle is stationary, a service braking force which is required to stop the vehicle by braking is calculated and is compared with the maximum parking braking force which can be produced by the parking brake, with an audible and/or visual warning signal being produced if the service braking force is greater than the parking braking force. This is because, in this case, the maximum parking braking force which can be produced by the parking brake is not sufficient to keep the vehicle stationary. Alternatively, the braking effect which takes place before the vehicle comes to rest can also be produced by the service brake with a service braking force which is at most as great as the maximum parking braking force which can be produced by the parking brake. It is therefore possible for the driver to tell whether the maximum parking braking force which can be produced by the parking brake is sufficient to prevent the vehicle from rolling. In a particular preferable embodiment, once the service brake has been released and the parking brake has been applied when the engine stops and the ignition switch switched off, at least a portion of the braking control of the EBS braking system is switched off. This results in energy-saving advantages, in order to prevent discharging of the vehicle battery.
According to one development, when the control element for the parking brake is operated, the service brake is applied in order to produce the state in which the vehicle is stationary, with the service brake being released and the parking brake being applied once the vehicle drive has been switched off. This has the advantage that the electronic braking system (EBS) can be used in a highly controllable manner for braking while driving, once again controlled by the control element for the parking brake, even though the actuator for operation of the parking brake has only an on-off functionality. If braking using the electronic braking system (EBS) is not possible, the parking brake can be activated immediately.